The resistance to the antibiotic is

The resistance to the antibiotic is not duemerely to the presence of the plasmid in thetransformed cells. The resistance gene on the plasmid must also be expressed, SO that the enzyme that detoxifies the antibiotic is synthesized. Expression of the resistance gene begins immediately after transformation. But it will be a few minutes before the cell contains enough of the enzyme to be able to withstand the toxic effects of the antibiotic.So the transformed bacteria should not be plated onto the selective medium immediately after the heat shock treatment. But it must be first placed in a small volume of liquid medium, in the absence of antibiotic, and incubated for a short time. Plasmid replication and expressioncan then get started, So that when the cells are plated out and encounter the antibiotic, they already have synthesized sufficient resistance enzymes to be able to survive.